Suppression of a defective alanyl-tRNA synthetase in Escherichia coli: A compensatory mutation to high alanine affinity

Abstract

Among temperature resistant revertants of a temperature sensitive E. coli alanyl-tRNA synthetase mutant a strain was found which contains an alanyl-tRNA synthetase with an additional mutation in the structural gene of the enzyme. This mutant enzyme has a 9 or 38 fold decreased K_m value for alanine compared to that of the thermolabile parental enzyme or to wild-type enzyme, respectively. The alaS gene maps just counterclockwise from recA on the E. coli map (94% cotransduction frequency). It appears that the enzyme's increased affinity for alanine is the mechanism of suppressing the temperature sensitive character of the cell. In addition, some coldsensitive temperature resistant revertants were found, where the cold-sensitive character mapped near strA, Presumably they are due to changes in ribosomal proteins as characterized by Ruffler et al. (1974).